The campaign headquarters for US Democratic presidential candidate Hillary Clinton has been evacuated after a white substance was discovered in an envelope, police say.

The substance was first discovered by two interns at a campaign office in Manhattan on Friday night and was taken to Clinton's Brooklyn headquarters, triggering the evacuation of the 11th floor, a New York Police Department spokesman said.

A total of four people were exposed to the substance, police said, though there have been no injuries or reported illnesses. Police are testing the substance and investigating the case.